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Size effects in composite materials.

Niklewicz, J; Sims, G D (2002) Size effects in composite materials. NPL Report. MATC(A)74

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The majority of property design data are obtained on small specimens for reasons of material availability and cost, fabrication cost and test machine capacity. Consequently, for design of relatively thick or large products, testing has to be carried out either on full-scale prototypes, or, in order to save both time and expense on small-scale models using the principles of dimensional analysis, or, the cheaper option, the data obtained on small coupon specimens must be used. Therefore, any discrepancies encountered whilst variations from small coupon to large product (i.e. any scale or size effects) needs to be both identified and understood. There is evidence from several researchers of size effects but these are based often on bending tests that have both complex and non-uniform stress distributions. In addition, there are multiple loading points that cause local variations in the stress field at these contact points. The opposing view is also present, when the coupon data is taken as a minimum value and higher results are expected for thicker components built using 160 mm thick material.
Hence, the work described here concentrates on axial compression and tension testing. The report describes several aspects of testing that are related to determining the properties of composites over arrange of sizes. It is clear that there exist many artefacts that can suggest size effects but when carefully analysed, there is little evidence of a Weibull weakest-link type size effect. Several steps are identified to minimise testing artefacts, such as :-
- to ensure that the same strain rate is used for all tests,
- to use 'distribution' ply stacking sequences,
- to use the optimised tab design described in this work,
- to use 90º and 45º as outer plies to reduce the tab stress concentration.

Item Type: Report/Guide (NPL Report)
NPL Report No.: MATC(A)74
Keywords: polymer matrix composites, reinforced plastics, size effects, scale effects, test methods, design data
Subjects: Advanced Materials
Advanced Materials > Composites
Last Modified: 02 Feb 2018 13:17
URI: http://eprintspublications.npl.co.uk/id/eprint/2205

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